Editing G2 phase (section)

=== Spatial regulation ===
In addition to the bistable and hysteretic aspects of cyclin B1-CDK1 activation, regulation of subcellular protein localization also contributes to the G2/M transition. Inactive cyclin B1-CDK1 accumulates in the cytoplasm, begins to be activated by cytoplasmic cdc25, and then is rapidly sequestered into the nucleus during prophase (as it is further activated). In mammals, cyclin B1/CDK1 translocation to the [[Nucleus (cell)|nucleus]] is activated by phosphorylation of five [[serine]] sites on cyclin B1's cytoplasmic retention site (CRS): S116, S26, S128, S133, and S147. In ''[[Xenopus laevis]]'', cyclin B1 contains four analogous CRS serine phosphorylation sites (S94, S96, S101, and S113) indicating that this mechanism is highly conserved. Nuclear export is also inactivated by phosphorylation of cyclin B1's [[nuclear export signal]] (NES). The regulators of these phosphorylation sites are still largely unknown but several factors have been identified, including [[extracellular signal-regulated kinases]] (ERKs), [[PLK1]], and CDK1 itself. Upon reaching some threshold level of phosphorylation, translocation of cyclin B1/CDK1 to the nucleus is extremely rapid. Once in the nucleus, cyclin B1/CDK1 phosphorylates many targets in preparation for mitosis, including [[histone H1]], [[nuclear lamins]], [[Centrosome|centrosomal proteins]], and [[Microtubule-associated protein|microtubule associated proteins (MAPs)]].

The subcellular localization of cdc25 also shifts from the cytosol to the nucleus during prophase. This is accomplished via removal of nuclear localization sequence (NLS)-obscuring phosphates and phosphorylation of the nuclear export signal. It is thought that the simultaneous transport of cdc25 and cyclin-B1/CDK1 into the nucleus amplify the switch-like nature of the transition by increasing the effective concentrations of the proteins.<ref name=":0" />